1. Field of the Invention
The present invention relates to a VoIP modem, and more specifically, to a VoIP modem that is capable of performing pulse dialing using only one relay.
2. Description of the Prior Art
With the growing popularity of high-speed Internet connections, it is now feasible for Voice over Internet Protocol (VoIP) phone calls to be made over the Internet. A subscriber line interface circuit (SLIC) is used to interface analog telephone signals with a VoIP modem. One main advantage of using the VoIP modem is VoIP phone calls are significantly less expensive than phone calls made over a public switched telephone network (PSTN). However, the voice quality of the VoIP phone calls is sometimes unstable, and, in some important situations, such as in an emergency, the user does not wish to encounter telephone service failure. Therefore, VoIP modems with PSTN backup capability have been developed. In such devices, if the user dials in VoIP mode a telephone number (such as 911) which is specifically pre-stored in the device, the number will be automatically redialed to PSTN to establish a phone call through PSTN instead of through VoIP network.
Please refer to FIG. 1. FIG. 1 is a functional block diagram of a VoIP modem dialing system 10 with PSTN backup capability according to the prior art. The system 10 comprises a VoIP modem 11 and a telephone set 12. The telephone set 12 communicates with a public switched telephone network (PSTN) 14 and a VoIP network 13 through the VoIP modem 11. The VoIP modem 11 comprises a subscriber line interface circuit (SLIC) 18 for interfacing analog telephone signals to digital signals, a relay 16 for selectively connecting the telephone set 12 with the PSTN 14 or with the SLIC 18, a DSP (Digital Signal Processor) 20 coupled to the SLIC 18 and to the relay 16 for controlling the relay 16 to connect the telephone set 12 with the PSTN 14 and with the SLIC 18, a DTMF (Dual Tone Multi-Frequency) coupling circuit 22 for coupling the DTMF redial signals from the SLIC 18 to the PSTN 14 when the user dials in VoIP mode a telephone number which matches a predetermined number in the device, and other circuits 21.
The relay 16 can be a mechanical relay, and can connect the telephone set 12 to either the PSTN 14 or the SLIC 18 at a time. The DSP 20 controls the relay 16 to switch back and forth according to control signals outputted from the DSP 20 to the relay 16.
However, most telephone sets are provided with tone-dialing mode and pulse-dialing mode. Unfortunately, the prior art VoIP modem 11 is only capable of making redialed PSTN phone calls using tone-dialing or dual tone multi-frequency (DTMF) dialing. The prior art VoIP modem 11 cannot provide pulse-dialing features.